Epoxy resins are widely used in protective coatings, high performance compositions, adhesives, and fiber-reinforced composites. Among these epoxy resins, those based on bisphenol-A currently play a leading role. In recent years, due to the escalated requirements for epoxy resins having improved physical and chemical properties many attempts have been carried out in order to synthesize more chemical, solvent and high temperature resistant, and durable epoxy resins.
The material containing bisphenol-S is of interest as an intermediate in the preparation of high temperature resistant thermoset and thermoplastic polymer for various end use applications. For instance, epoxy resin based on bisphenol-S has the advantage of resistance to deformation by heat and thermal stability. Such improved epoxy resins from bisphenol-S have other advantages in briefer gel periods and more rapid development of mechanical properties in cured systems, better resistance to organic solvent attack, increased dimensional stability and better wetting of glass reinforcement. [See J. C. Spitsbergen, P. Loewrigkeit, C. Bluestein, J. Sugarman and W. L. Lauze, 26th annu. Techn. Conf. Reinforced Plastics/Composites Division, The Society of the Plastics Industry, Inc. Section 19-C, P.1, (1971); and D. J. Liaw and W. C. Shen, Angew. Makromol. Chem. 199, 171 (1992)] Although epoxy resins based on bisphenol-S have better thermal resistance and mechanical strength than those of epoxy resins based on bisphenol-A, the bisphenol-S type epoxy resins suffer from lack of flexibility in comparison with the bisphenol-A type epoxy resins. Therefore, even though U.S. Pat. No. 3,790,602 discloses an accelerated process for preparing bisphenol-S diglycidyl ethers, the epoxy resins based on bisphenol-S are not produced in large scale up to the present, and there is still a need for epoxy resins having improved flexibility and comparable performance in mechanical strength at the same time.
An object of the present invention is to provide novel bisphenol-S type epoxy resins having improved flexibility and comparable mechanical performance compared to the prior art epoxy resins.